Development of a Magnetostrophic Geodynamo

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A dynamo is a device that converts mechanical energy into electrical energy. It does this by moving an electrical conductor across a magnetic field. This induces an electrical current which creates its own induced magnetic field. The dynamo becomes self-excited when the induced and inducing magnetic fields are the same. A similar self-excited process creates Earth's magnetic field. The associated electric currents flow in the Earth's core, a nearly spherical mass of liquid iron, of radius 2,200 miles, surrounding the Earth's center. Because of this conducting fluid's motion, the Earth's core is a gigantic electric generator, called 'the geodynamo'. A dynamo also needs a power source to maintain its motion and production of electricity despite heat losses through electrical resistance. For the geodynamo the power source is thought to be due to the buoyancy of density differences in the core. The power is expended not only in maintaining the magnetic field but also in overcoming the frictional forces opposing the fluid motion. Current numerical simulations assume that the frictional energy losses are comparable with those from electrical resistance, but it is generally agreed that in reality they are very much less. A dynamo that ignores frictional losses is termed magnetostrophic, and the investigators have recently revived an old idea of the plasma physicist, Bryan Taylor, that has led them to a new numerical procedure through which they have already derived two-dimensional solutions of the magnetostrophic equations. The investigators believe that, when they have made the solutions three-dimensional (a complicated step), they will become the new paradigm for future simulations of magnetic field creation and lead to a better understanding of how the Earth and other planets create their magnetism.The north-seeking property of the magnetic compass needle shows that the Coriolis force strongly influences core motion. This force is balanced primarily by buoyancy and the Lorentz force, which is the force exerted on a current-carrying conductor by the magnetic field. Taylor showed in 1953 how the resulting simplified dynamical balance could be achieved, so determining the fluid velocity as a functional of the magnetic field. To close the regenerative loop, it is necessary to determine the magnetic field from the fluid velocity, but that is the well-known and well-understood kinematic dynamo process. The way ahead seemed simple in 1953 but all the determined attempts made since then to implement Taylor's suggestion failed, prior to that of the investigating team this year. The model was greatly simplified by assuming symmetry with respect to the Earth's rotation axis. The well-known alpha-effect was invoked as the power source. This also made Cowling's theorem, that otherwise would have ruled out axisymmetric solutions, inapplicable. To convince the dynamo community that this success is meaningful, the team plans to abandon axisymmetry, remove the alpha-effect, and rely on buoyancy to supply the required power. The investigators believe that their results will be an eye-opener for the dynamo community, and will become the basis for models of naturally-occurring dynamos in planets and stars that are more realistic than today's simulations which assume a viscous friction that is very much too large.